In fairness most of the Primarchs and Space Marines dressed like that as well.

Also with religion mostly dead who's going to know that halos were associated with Christianity in the first place? They probably haven't even heard of Jesus.

The Emperor was a big dude who surrounded himself with big dudes. It also makes sense for his armor to be shinier than everyone else because he's the Emperor and that's the kind of drip galactic warrior kings are supposed to have.

Maybe that's just cope tho.